The dispute revolves around the interpretation of four words in the law. Obamacare opponents argue the law limits health insurance subsidies to people living where a healthcare exchange had been “established by a state” because that is what the Affordable Care Act (ACA) technically says. If the Supreme Court agrees with that literal reading of those four words, 8.2 million people in America will lose their health insurance, according to the amicus brief filed by the American Public Health Association.

That brief went on to say that the “interrelationship between insurance coverage, healthcare access, and population health” means that a loss of insurance of that magnitude translates to 9,800 additional deaths every single year.

“Nothing in the ACA requires these terrible health outcomes,” the brief stated.

This is the second time the high court has heard a challenge to Obamacare. In 2012 the Supreme Court upheld another key part of Obamacare — the requirement that most people buy health insurance or pay a penalty. To make insurance affordable, the law stipulated that states would set up subsidized exchanges. If the states could not set up exchanges, the law said the federal government would create one for the state.

In fact, the majority of states failed to establish their own exchanges, forcing the federal government to step in. Now, opponents of the law — four Virginia residents who don’t want to buy health insurance — claim the law doesn’t authorise subsidies for people buying insurance from exchanges set up by the federal government.

They point to text of the law specifying that subsidized insurance is available through “exchanges established by the state.” Read literally, that would exclude exchanges the federal government established on behalf of states.

But the federal government says those four words must be read in the context of the law. In a brief to the Supreme Court, the US Secretary of Health and Human Services points out that subsidized insurance is the key to making the entire law work. Those four words shouldn’t be read in a vacuum, the brief said.

While the Supreme Court hears the case Wednesday, it could be months before a decision is reached. Chief Justice John Roberts, a conservative, was the surprise swing vote the last time the court ruled on Obamacare and could play a decisive role in the case again.